Optical fiber draw furnace system and method

ABSTRACT

An optical fiber draw system and method of operating thereof. The method includes positioning a downfeed handle for supporting an optical fiber preform within a furnace such that the downfeed handle is movable within the furnace. The method further includes operating one or more heating elements to thermally heat at least a portion of an upper muffle extension disposed within the furnace, the one or more heating elements being moveable with the downfeed handle.

This Application is a division of U.S. patent application Ser. No.17/155,201, filed on Jan. 22, 2021, which claims the benefit of priorityunder 35 USC § 119(e) from U.S. Provisional Patent Application Ser. No.62/965,473 filed on Jan. 24, 2020 which are incorporated by referenceherein in their entirety.

FIELD OF THE DISCLOSURE

The present invention is generally directed to systems and methods foroperating an optical fiber draw furnace, and more particularly relatesto systems and methods for operating an optical fiber draw furnace whileheating a downfeed handle within the draw furnace.

BACKGROUND OF THE DISCLOSURE

Optical fibers are generally manufactured to include an inner glass coresurrounded by a glass cladding and multiple layers of coatings toprovide sufficient bending and damage resistance. Conventionaltechniques and manufacturing processes for producing optical fibersinclude drawing an optical fiber from a preform. The preform is formedof consolidated silica glass, which includes a series of concentricregions of silica glass that differ in the level or type of dopant.Control of the spatial distribution, concentration, and/or type ofdopant in the preform creates regions that differ in refractive index.These differences in refractive index define different functionalregions in the produced optical fiber (e.g. core vs. cladding, low indexdepressions, tailored index profiles).

Drawing of the preform is typically performed in a draw furnace andinvolves melting and stretching the preform to achieve a target opticalfiber diameter. Various properties, including furnace temperature,preform position, and pulling speed, are controlled in order to producean optical fiber with a constant diameter. For example, temporalvariation in the temperature of the furnace can cause variation in thecooling rate of the preform during the drawing procedure, resulting inan uneven and irregular optical fiber diameter.

Furthermore, unsteady convection of gases within the draw furnace canlead to an uneven and irregular optical fiber diameter. Inert processgas is typically introduced into an upper portion of the draw furnace inorder to prevent ambient air from entering the furnace. Ambient air canreact with components of the draw furnace, causing unwanted oxidation.But, flow instabilities in the process gas at the upper portion of thefurnace can affect uniform drawing of the preform. More specifically,flow instabilities in the upper, not actively heated, portion of thefurnace are propagated downward in the furnace, towards the neckdownregion of the preform. This can affect the heat transfer between theprocess gas and the neckdown region of the preform, which in turn leadsto fluctuations in cooling rate of the preform, resulting in diameterfluctuations of the drawn optical fiber.

Conventional inert process gases include nitrogen and argon. But, thesegases can cause the undesired flow instabilities in the upper portion ofthe draw furnace. Helium gas is known to reduce any unsteady convectionin a draw furnace and, thus, has been used in place of nitrogen andargon to provide more uniform diameters in the drawn optical fibers.However, helium is a nonrenewable resource recovered as a byproduct fromnatural gas wells. The price of helium is projected to increase in thefuture, thus increasing the need to use other gases in the draw furnace.There is therefore a need to provide systems and methods for operating adraw furnace without having to necessarily use helium while stillmaintaining a steady gas convection within the draw furnace.

SUMMARY OF THE DISCLOSURE

According to one embodiment, a method of operating an optical fiber drawfurnace is provided. The method includes positioning a downfeed handlefor supporting an optical fiber preform within a furnace such that thedownfeed handle is movable within the furnace. The method also includesthe step of operating one or more heating elements to thermally heat atleast a portion of an upper muffle extension disposed within thefurnace, the one or more heating elements being moveable with thedownfeed handle.

According to another embodiment, an optical fiber draw furnace system isprovided. The system includes a muffle comprising an upper muffleextension and forming an inner cavity. The system also includes adownfeed handle and an upper heater. The downfeed handle is moveablypositioned within the inner cavity. Furthermore, the upper heaterincludes one or more heating elements moveable with the downfeed handlewithin the inner cavity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating a draw furnace assemblyaccording to embodiments of the present disclosure;

FIG. 2 is a schematic diagram illustrating the draw furnace assembly ofFIG. 1 in another position;

FIG. 3 is a schematic diagram illustrating the draw furnace assembly ofFIGS. 1 and 2 in another position;

FIG. 4 is a schematic diagram illustrating the draw furnace assembly ofFIGS. 1-3 in another position;

FIG. 5 is a schematic diagram illustrating a draw furnace assemblyaccording to embodiments of the present disclosure;

FIG. 6A illustrates temperature contour plots of the draw furnaceassembly according to embodiments of the present disclosure and ofcomparative draw furnace assemblies;

FIG. 6B illustrates stream function contour plots of the gas in the drawfurnace assembly according to embodiments of the present disclosure andof comparative draw furnace assemblies. Closed lines of stream functionindicate cellular flow;

FIG. 7 illustrates plots of temperature vs. time in the upper portion ofthe draw furnace assembly according to embodiments of the presentdisclosure and of comparative draw furnace assemblies;

FIG. 8 illustrates plots of gas pressure vs. time in the upper portionof the draw furnace assembly according to embodiments of the presentdisclosure and of comparative draw furnace assemblies;

FIG. 9 illustrates plots of temperature vs. time in the neckdown regionof the preform within the draw furnace assembly according to embodimentsof the present disclosure and of comparative draw furnace assemblies;

FIG. 10 illustrates plots of gas pressure vs. time in the neckdownregion of the preform within the draw furnace assembly according toembodiments of the present disclosure and of comparative draw furnaceassemblies;

FIG. 11 illustrates a plot of temperature vs. time in the neckdownregion of the preform for draw furnace assemblies heated to differenttemperatures;

FIG. 12 illustrates a plot of amplitude vs. frequency in the neckdownregion of the preform for the draw furnace assemblies of FIG. 11 ; and

FIG. 13 illustrates a plot of gas pressure vs. time in the neckdownregion of the preform for draw furnace assemblies heated to differenttemperatures.

DETAILED DESCRIPTION

Additional features and advantages of the disclosure will be set forthin the detailed description which follows and will be apparent to thoseskilled in the art from the description, or recognized by practicing thedisclosure as described in the following description, together with theclaims and appended drawings.

As used herein, the term “and/or,” when used in a list of two or moreitems, means that any one of the listed items can be employed by itself,or any combination of two or more of the listed items can be employed.For example, if a composition is described as containing components A,B, and/or C, the composition can contain A alone; B alone; C alone; Aand B in combination; A and C in combination; B and C in combination; orA, B, and C in combination.

In this document, relational terms, such as first and second, top andbottom, and the like, are used solely to distinguish one entity oraction from another entity or action, without necessarily requiring orimplying any actual such relationship or order between such entities oractions.

It will be understood by one having ordinary skill in the art thatconstruction of the described disclosure, and other components, is notlimited to any specific material. Other exemplary embodiments of thedisclosure disclosed herein may be formed from a wide variety ofmaterials, unless described otherwise herein.

It is also important to note that the construction and arrangement ofthe elements of the disclosure, as shown in the exemplary embodiments,is illustrative only. Although only a few embodiments have beendescribed in detail in this disclosure, those skilled in the art whoreview this disclosure will readily appreciate that many modificationsare possible (e.g., variations in sizes, dimensions, structures, shapesand proportions of the various elements, values of parameters, mountingarrangements, use of materials, colors, orientations, etc.) withoutmaterially departing from the novel and nonobvious teachings andadvantages of the subject matter recited. For example, elements shown asintegrally formed may be constructed of multiple parts, or elementsshown as multiple parts may be integrally formed, the operation of theinterfaces may be reversed or otherwise varied, the length or width ofthe structures, and/or members, or connectors, or other elements of thesystem, may be varied, and the nature or number of adjustment positionsprovided between the elements may be varied. It should be noted that theelements and/or assemblies of the system may be constructed from any ofa wide variety of materials that provide sufficient strength ordurability, in any of a wide variety of colors, textures, andcombinations. Accordingly, all such modifications are intended to beincluded within the scope of the present disclosure. Othersubstitutions, modifications, changes, and omissions may be made in thedesign, operating conditions, and arrangement of the desired and otherexemplary embodiments without departing from the spirit of the presentdisclosure.

Reference will now be made in detail to the present preferredembodiments of the disclosure, examples of which are illustrated in theaccompanying drawings. Whenever possible, the same reference numberswill be used throughout the drawings to refer to the same or like parts.

Referring now to FIG. 1 , an exemplary optical fiber draw furnace systemis shown generally designated by reference numeral 10, according to oneexample. Draw furnace 10 includes a muffle 20 disposed within an outercan 30. A downfeed handle 40 is moveably positioned within a handlecavity of muffle 20 to support an optical fiber preform 50. As discussedfurther below, an upper heater comprised of one or more heating elementsis coupled to and moveable with downfeed handle 40. The heating elementshelp to provide a more uniform temperature and steady convection ofgases within draw furnace 10.

Muffle 20 comprises a first end portion 24 and a second end portion 25,as shown in FIG. 1 . Second end portion 25 forms an upper muffleextension 23, which extends downward along a predetermined length ofmuffle 20. A top hat 21 is positioned above upper muffle extension 23and provides sealing and motion capabilities, as is known in the art. Asshown in FIG. 1 , muffle 20 and top hat 21 form an inner cavity 27through which downfeed handle 40 is moveably disposed. As discussedfurther below, cavity 27 includes a furnace cavity 22 at a first end ofcavity 27. Furthermore, a handle cavity 29 may form a portion of cavity27 that is disposed between downfeed handle 40 and muffle 20. Asdownfeed handle 40 moves within cavity 27, handle cavity 29 may comprisedifferent portions of cavity 27. For example, as downfeed handle 40moves lower within cavity 27 so that a greater portion of downfeedhandle 40 is disposed within muffle 20, handle cavity 29 also increasesin length. An elastomer seal 26 may provide an airtight connectionbetween downfeed handle 40 and upper muffle extension 23.

Muffle 20 and/or upper muffle extension 23 may be composed of arefractory material and/or a refractory metal such as, for example,graphite, zirconia, binders, alumina, mullite, quartz, silicon carbide,silicon nitride, and/or combinations thereof. Therefore, muffle 20and/or upper muffle extension 23 may be formed of carbon, which canreact with ambient air and combust. Additionally, muffle 20 and uppermuffle extension 23 may be a single component or formed of two or moreseparate components. As shown in FIG. 1 , muffle 20 and upper muffleextension 23 may have a substantially uniform inner diameter. It is alsocontemplated that muffle 20 and upper muffle extension 23 may havedifferent inner diameters. In some embodiments, the inner diameter ofmuffle 20 and/or upper muffle extension 23 may vary along the length ofthe component.

A lower heater 60 is disposed within outer can 30 adjacent to the firstend portion 24 of muffle 20. Lower heater 60 may be thermally coupled tomuffle 20 to create a hot zone within furnace cavity 22. The hot zonemay have a temperature of from about 1800° C. to about 2100° C. In someembodiments, the hot zone may have a temperature of about 1800° C.,about 1900° C., about 2000° C., or about 2100° C., or any range havingany two of these values as endpoints. As will be explained in greaterdetail below, the heat of the hot zone is sufficient to decrease theviscosity of preform 50. In some embodiments, lower heater 60 maycomprise an induction coil.

Furthermore, muffle 20 and/or upper muffle extension 23 are configuredto retain heat within draw furnace 10, as well as protect othercomponents from excess temperatures. For example, muffle 20 and/or uppermuffle extension 23 may have insulating properties sufficient tomaintain the elevated temperature of the hot zone within furnace cavity22. It is also contemplated that, for example, an insulation 65surrounds muffle 20. As shown in FIG. 1 , insulation 65 may be disposedbetween muffle 20 and the induction coil of lower heater 60 and disposedbetween upper muffle extension 23 and outer can 30. Therefore,insulation 65 may extend in length from lower heater 60 to upper muffleextension 23 such that it is disposed around upper muffle extension 23.In some embodiments, insulation 65 is a graphite insulation.

Although muffle 20 and/or upper muffle extension 23 may generally begood insulators, oxidation may still occur at elevated temperatures.Therefore, one or more process gases may be inserted or injected intodraw furnace 10 to prevent oxidation of these components. As discussedfurther below, the process gases may include an inert gas such as, forexample, nitrogen, argon, helium, and/or a combination of these gases.

Outer can 30 may include one or more gas inlet ports to inject theprocess gas into cavity 27. For example, as shown in FIG. 1 , outer can30 includes a first gas inlet port 70, a second gas inlet port 72, and athird gas inlet port 74. First gas inlet port 70 is disposed in uppermuffle extension 23, second gas inlet port 72 is disposed in the outercan 30 near lower heater 60, and third gas inlet ports 74 is disposed ata bottom of muffle 20. The process gas may be injected between an outerwall of muffle 20 and an inner wall of can 30. The process gas may alsobe injected in cavity 27, as shown in FIG. 1 . As discussed furtherbelow, the process gas is injected into draw furnace 10 to ensure thatambient air does not enter draw furnace 10 during a drawing procedure.Therefore, oxygen from the ambient air is prevented from reacting with,for example, the carbon of muffle 20.

Preform 50 may be attached to and hung from downfeed handle 40 using asupport member 80. It is contemplated that support member 80 is acomponent of downfeed handle 40, or is a separate component coupled todownfeed handle 40. Support member 80 may have substantially the sameouter diameter as downfeed handle 40. Therefore, a gap between an outerdiameter of downfeed handle 40 and an inner diameter of muffle 20 may besubstantially equal to a gap between an outer diameter of support member80 and the inner diameter of muffle 20. Support member 80 is configuredto support preform 50. In some embodiments, support member 80 is a pieceof glass welded to downfeed handle 40. Additionally or alternatively,support member 80 may include a slot to which preform 50 is attached.However, it is also contemplated that any suitable configuration may beused to attach preform 50 to downfeed handle 40.

Downfeed handle 40 may be composed of, for example, quartz glass,graphite, silicon nitride, silicon carbide or silicon carbide coatedgraphite, and downfeed handle 40 has an outer diameter smaller than aninner diameter of muffle 20. Thus, downfeed handle 40 (along withsupport member 80) is moveable within muffle 20 and top hat 21 along alongitudinal direction of these components (e.g., up and down).Additionally, downfeed handle 40 (along with support member 80) may bemoveable within muffle 20 and top hat 21 in a radial direction of thesecomponents (e.g., left and right; front and back) and may be rotatablewithin these components. Preform 50, when attached to downfeed handle40, may move with downfeed handle 40 within muffle 20 and top hat 21.For example, during a drawing process, downfeed handle 40 may movelongitudinally within cavity 27 as preform 50 is consumed. As shown inFIGS. 1-4 , downfeed handle 40 moves along a length of muffle 20 duringa drawing process.

As preform 50 moves with downfeed handle 40 within muffle 20 and islowered towards lower heater 60, an optical fiber may be drawntherefrom. Preform 50 may be composed of any well-known glass or othermaterial and may be doped suitable for the manufacture of opticalfibers. In some embodiments, preform 50 includes a core and a cladding.As preform 50 reaches the hot zone of lower heater 60, the viscosity ofpreform 50 is lowered such that an optical fiber may be drawn frompreform 50. As preform 50 is continuously consumed during the drawingprocess, downfeed handle 40 may be continuously lowered such that newportions of preform 50 are exposed to the hot zone created by lowerheater 60. The optical fiber is drawn from preform 50 out through abottom of draw furnace 10 and may be wound onto a spool. In someembodiments, the optical fiber has a diameter of about 125 microns.

As discussed above, process gas is injected into muffle 20 during thedrawing of preform 50. More specifically, during the drawing process, adoor 76 is opened and process gas is injected into cavity 27 through gasinlet ports 70 and/or 72. The process gas injected into first gas inletport 70 flows down through cavity 27, along the length of preform 50 andthrough furnace cavity 22, and into a lower muffle extension 90. Thenthe process gas exits through door 76. This flow path of the process gasis used to prevent ambient air from entering muffle 20 during thedrawing process.

Gas inlet port 72 is used when lower heater 60 is powered on to heatpreform 50. Process gas injected into gas inlet port 72 may flow upwardwithin outer can 30 and exit draw furnace 10 near second end portion 25of muffle 20. The process gas injected into gas inlet port 72 may beused as a purge gas to ensure that air is not present in outer can 30,which could react with insulation 65.

Furthermore, process gas is injected into gas inlet port 74 duringloading and unloading procedures of preform 50. During these procedures,door 76 is closed and the process gas injected into gas inlet port 74flows upward within cavity 27. This prevents air from entering a topportion of draw furnace 10 during the loading and unloading procedures.

In traditional draw furnace systems, the process gas is subject to flowinstabilities as it flows within a draw furnace during a drawingprocedure. As discussed above, such flow instabilities in the processgas can cause an uneven and irregular diameter in the drawn opticalfiber. The flow instabilities arise from unsteady natural convection dueto density stratification in the muffle cavity and due to the flow ofthe inert gas, which are propagated down through the muffle. These flowinstabilities ultimately affect the heat transfer between the processgas and a draw root of an optical fiber preform. More specifically, theflow instabilities are manifested as temperature variations, pressurevariations, and mass flow variations that are translated to the drawroot and cause changes in viscosity of the preform. The temperature,pressure, and mass flow variations lead to fluctuations in the heatingand cooling of the draw root, resulting in fluctuations of the diameterof the optical fiber drawn from the preform (e.g., due to changes in theamount of material which may be pulled from the optical fiber preformfor a given speed and tension).

The flow instabilities, or unsteadiness, of the process gas may bequantified by a Grashof (Gr) number. The Gr number can be interpretedphysically as the ratio of the buoyancy forces to the viscous forces ofa gas system. When buoyancy forces become significantly larger than theviscous forces, flow becomes unstable and temporally-variant. TheGrashof number is expressed numerically by equation (1):

$\begin{matrix}{{Gr} = \frac{{\mathcal{g}\beta}L_{c}^{3}\Delta T}{v^{2}}} & (1)\end{matrix}$

where, g is the gravitational acceleration, β is the coefficient ofthermal expansion of the process gas, L_(c) is the characteristic length(e.g., the length of the space in which the gas is disposed), ΔT is thetemperature difference (e.g. as measured proximate the draw root of theoptical fiber preform) and v is the kinematic viscosity of the processgas.

As discussed above, some draw furnaces may use helium because helium hasa high kinematic viscosity. As can be seen from equation (1), a highkinematic viscosity of the process gas may lead to a lower Grashofnumber, which results in steady, time-invariant natural convection flow.Stated another way, process gases with higher kinematic viscosity resistunsteady buoyancy-driven flow. Furthermore, process gases with lowerconvective flow are less likely to cause unsteady flow behavior in acavity of a muffle. Thus, the higher the kinematic viscosity of theprocess gas, the more resistant to buoyancy-driven convective flow inthe process gas, thus decreasing or preventing unsteady flowinstabilities in the muffle. Generally, a Grashof number of from about7,000 or less, 8,000 or less, 9,000 or less, 10,000 or less, 11,000 orless, or 12,000 or less results in stable, time-invariant flow while aGrashof number of greater than about 13,000 results in unsteady,time-variant flow.

Referring to FIG. 1 , an upper heater comprised of one or more heatingelements 46 may be coupled to downfeed handle 40 to adjust thetemperature difference in muffle 20 to reduce the Grashof number and topromote stable flow of the process gas, thereby reducing diametervariation in the drawn optical fiber. Thus, the one or more heatingelements 46 may allow the use of process gases other than helium whilestill obtaining a desirably low Grashof number and stable flow. Forexample, heating elements 46 in draw furnace 10 may provide a Grashofnumber in the range of about 800 to about 1200 when using either argonor nitrogen as the process gas.

Heating elements 46 may include, for example, wound resistance heaters,band heaters, and/or immersion/bar heaters, as is well known in the art.

As shown in FIG. 1 , heating elements 46 may be powered (i.e., turned toan on position) to form different heating zones on downfeed handle 40.For example, different heating elements are powered to form a firstheating zone 41, a second heating zone 42, a third heating zone 43, anda fourth heating zone 44 of downfeed handle 40. Although the embodimentof FIG. 1 shows four heating zones, it is also contemplated that more orless heating zones may be used. For example, heating elements 46 mayform, for example, one, two, five, six, seven, eight, or ten heatingzones. Each heating zone may be heated and powered on independently fromthe other heating zones.

The zones 41, 42, 43, 44 may be disposed on an inner surface of downfeedhandle 40 and extend along an entire inner circumference of downfeedhandle 40. However, it is also contemplated that the zones may extendfor less than the entire inner circumference of downfeed handle 40.Furthermore, zones 41, 42, 43, 44 may each have a length of about 8 in.to 12 in. One or more zones may be the same or different size in lengthas one or more other zones.

Heating elements 46 may heat one or more heating zones 41, 42, 43, 44 ofdownfeed handle 40, which in turn heats the process gas disposed withinhandle cavity 29. As discussed above, handle cavity 29 is the portion ofcavity 27 that is disposed between downfeed handle 40 and muffle 20.Handle cavity 29 may comprise different portions of cavity 27 asdownfeed handle 40 moves within draw furnace 10. The heating of theprocess gas disposed in handle cavity 29 may then heat a portion ofupper muffle extension 23 that surrounds the heated downfeed handle 40(and, therefore, that surrounds handle cavity 29). Thus, heatingelements 46 heat the portion of upper muffle extension 23 that surroundsthe heated downfeed handle 40.

The heating of the process gas disposed within handle cavity 29increases the temperature of the process gas, which reduces temperaturesgradients in both vertical and radial directions and increases thekinematic viscosity of the process gas. Such reduction of temperaturegradients and increase of the kinematic viscosity results in increasedstability of the flow of the process gas. As discussed above, flowinstabilities in a draw operation may arise from the unsteady,buoyancy-driven flow of the process gas. The heat from downfeed handle40 reduces/prevents such flow instabilities in draw furnace 10. Heliumcan then be replaced with argon or nitrogen as the process gas.

The temperature of the process gas in handle cavity 29 may be increasedby about 450 to 750° C. due to heating elements 46. As shown in FIG. 1 ,insulation 65 around upper muffle extension 23 may help to maintain theincreased gas temperature within handle cavity 29.

As shown in FIG. 1 , heating elements 46 may be disposed inside of(e.g., on an inner wall surface) of downfeed handle 40. Thus, heatingelements 46 are disposed radially inward of muffle 20 (including uppermuffle extension 23) and of outer can 30. However, it is alsocontemplated that heating elements 46 may be disposed outward ofdownfeed handle 40 and/or muffle 20. In some embodiments, heatingelements 46 may be disposed on an outer wall surface of downfeed handle40 or embedded within the walls of downfeed handle 40. Heating elements46 may be disposed anywhere on draw furnace 10 such that they are ableto heat the process gas in the annular space between downfeed handle 40and upper muffle extension 23 to reduce temperature gradients of theprocess gas. However, providing heating elements 46 on downfeed handle40, rather than on a wall of muffle 20, provides the benefits ofreducing power consumption due to a reduction in heat loss to the uppercan, reducing design constraints by simplifying power and thermocouplewiring constraints, simplifying the sealing design of the muffle, andallowing heating of the handle to be utilized without modifying thefurnace proper. Heating elements 46 may comprise a plurality of heatingelements vertically positioned along a length of downfeed handle 40.

Heating elements 46 may be coupled to downfeed handle 40 such thatheating elements 46 are moveable with downfeed handle 40 within muffle20. Heating elements 46 may also sequentially heat zones 41, 42, 43, 44.For example, and as discussed further below, heating elements 46 maysequentially heat the zones as downfeed handle 40 (and, thus, heatingelements 46) move longitudinally within muffle 20.

In some embodiments, heating elements 46 may only heat a portion ofdownfeed handle 40 that is disposed within upper muffle extension 23.Thus, a portion of downfeed handle 40 that is disposed exterior of uppermuffle extension 23 (for example, within top hat 21) may not be heatedby heating elements 46 even when at least some heating elements 46 arepowered in an on position. This portion of downfeed handle 40 that wasinitially exterior of upper muffle extension 23, and not heated, maybecome heated by heating elements 46 as downfeed handle 40 movesdownward within muffle 20 (i.e., towards lower heater 60) such that thisportion of downfeed handle 40 is now disposed within upper muffleextension 23.

The portion(s) of downfeed handle 40 that are heated by heating elements46 may be heated to a temperature ranging from about 200° C. to about1200° C., from about 400° C. to about 1000° C., from about 600° C. toabout 900° C., from about 700° C. to about 850° C., or about 800° C.Therefore, each zone 41, 42, 43, 44 of downfeed handle 40 may beseparately and independently heated to a temperature within thesedisclosed ranges. One or more zones may be heated to a differenttemperature than one or more other zones. It is also contemplated thatall zones 41, 42, 43, 44 are heated to the same temperature.

In some embodiments, one or more zones 41, 42, 43, 44 may be heated witha temperature gradient in the particular zone. Thus, for example, firstzone 41 may be heated such that a top portion of the zone (further fromlower heater 60) is heated to a higher temperature than a bottom portionof the zone (closer to lower heater 60) with a gradient provided betweenthese two portions.

A control unit (not shown) may be coupled to heating elements 46 inorder to monitor and regulate the temperature of each zone 41, 42, 43,44 and the temperature of the process gas within handle cavity 29. Forexample, one or more sensors, such as thermocouples, may be coupled tothe control unit to monitor and regulate the temperatures. The sensorsmay help to provide closed loop temperature control and thermaltemperature gradient management.

As downfeed handle 40 moves within muffle 20 closer to lower heater 60,zones 41, 42, 43, 44 may be sequentially heated by heating elements 46.The sequential heating of the zones may occur as more and more portionsof preform 60 are consumed by the drawing process. For example, in afirst position of downfeed handle 40, the heating elements 46 may be inan off position such that none of the zones 41, 42, 43, 44 are heated.The first position may be used, for example, during loading andunloading of preform 50. In this first position, each of zones 41, 42,43, 44 may be disposed exterior of upper muffle extension 23.

In one embodiment, heating elements 46 may heat first zone 41 afterdownfeed handle 40 is lowered to a second position within cavity 27 suchthat first zone 41 is disposed, at least in part, within upper muffleextension 23. At this time, the remaining zones 42, 43, 44 are each notheated by heating elements 46 and are disposed, at least in part,exterior of upper muffle extension 23. FIG. 1 depicts the secondposition of downfeed handle 40 in which zone 41 is heated by heatingelements 46. As shown in FIGS. 1-4 , first zone 41 may be disposedclosest to lower heater 60 (when downfeed handle 40 is disposed withinmuffle 20) of all the zones.

Downfeed handle 40 may then move from the second position to a thirdposition by moving lower (e.g., relatively closer to lower heater 60)such that both first zone 41 and second zone 42 are disposed, at leastin part, within upper muffle extension 23. At this time, second zone 42may now be heated. Therefore, second zone 42 is heated after heatingfirst zone 41. Downfeed handle 40 may move from the second position tothe third position simultaneously as additional preform 50 is consumedby the drawing process. At the time of the third position of downfeedhandle 40, the remaining zones 43, 44 are each not heated by heatingelements 46 and are disposed, at least in part, exterior of upper muffleextension 23. FIG. 2 depicts the third position of downfeed handle 40 inwhich zones 41, 42 are both heated by heating elements 46. As also shownin FIGS. 1-4 , first zone 41 is located relatively closer to lowerheater 60 (when downfeed handle 40 is disposed within muffle 20) thansecond zone 42.

Downfeed handle 40 may then move from the third position to a fourthposition by moving lower (e.g., relatively closer to lower heater 60)such that first zone 41, second zone 42, and third zones 43 are eachdisposed, at least in part, within upper muffle extension 23. At thistime, third zone 43 may now be heated. Therefore, third zone 43 isheated after heating first and second zones 41, 42. Downfeed handle 40may move from the third position to the fourth position simultaneouslyas additional preform 50 is consumed by the drawing process. At the timeof the fourth position of downfeed handle 40, the remaining zone 44 isnot heated by heating elements 46 and is disposed, at least in part,exterior of upper muffle extension 23. FIG. 3 depicts the fourthposition of downfeed handle 40 in which zones 41, 42, 43 are all heatedby heating elements 46. As also shown in FIGS. 1-4 , first and secondzones 41, 42 are both located relatively closer to lower heater 60 (whendownfeed handle 40 is disposed within muffle 20) than third zone 43.

Downfeed handle 40 may then move from the fourth position to a fifthposition by moving lower (e.g., relatively closer to lower heater 60)such that first zone 41, second zone 42, third zone 43, and fourth zone44 are each disposed, at least in part, within upper muffle extension23. At this time, fourth zone 44 may now be heated. Therefore, fourthzone 44 is heated after heating first, second, and third zones 41, 42,43. Downfeed handle 40 may move from the fourth position to the fifthposition simultaneously as additional preform 50 is consumed by thedrawing process. FIG. 4 depicts the fifth position of downfeed handle 40in which zones 41, 42, 43, 44 are all heated by heating elements 46. Asshown in FIGS. 1-4 , first, second, and third zones 41, 42, 43 are alllocated relatively closer to lower heater 60 (when downfeed handle 40 isdisposed within muffle 20) than fourth zone 44.

Downfeed handle 40 may then move from the fifth position to apredetermined position by moving lower (e.g., relatively closer to lowerheater 60) within muffle 20. Downfeed handle 40 may move from the fifthposition to the predetermined position simultaneously as additionalpreform 50 is consumed by the drawing process. In some embodiments, thefifth position may be the predetermined position such that downfeedhandle 40 does not need to move in order to go from the fifth positionto the predetermined position. The predetermined position may be aposition relative to lower heater 60. When downfeed handle 40 is in thepredetermined position, as shown in FIG. 4 , the power of one or moreheating elements 46 corresponding to first zone 41 may be reduced. Forexample, the power of the one or more heating elements 46 correspondingto first zone 41 may be reduced by about 10%, about 20%, about 30%,about 40%, about 50%, about 60%, about 70%, about 80%, about 90%, orabout 100%. When downfeed handle 40 is in the predetermined position andthe one or more heating elements 46 corresponding to first zone 41 arereduced, the heating elements 46 corresponding to the remaining zones42, 43, 44 may be maintained at their power levels. Thus, in someembodiments, the reduced power of first zone 41 may be lower than thepower applied to each of second, third, and fourth zones 42, 43, 44.

When downfeed handle 40 is in the predetermined position, the reductionin power to first zone 41 may be compensated for by the proximity of thezone to lower heater 60. For example, heating elements 46 correspondingto first zone 41 may be reduced in power as first zone 41 moves closerto the heat of lower heater 60. Therefore, in some embodiments, thetemperature of first zone 41 may not decrease as the power of heatingelements 46 is reduced due to the additional heat from lower heater 60.

It is also contemplated that downfeed handle 40 may move in any orderbetween the first, second, third, fourth, fifth, and predeterminedpositions. Therefore, for example, in some embodiments, downfeed handle40 may move from the third position to the predetermined position.

In some embodiments, the heating elements 46 corresponding to both firstzone 41 and second zone 42 may be reduced in power when downfeed handle40 is in the predetermined position. It is also contemplated that theheating elements 46 corresponding to first, second, and third zones 41,42, 43 are reduced in power when downfeed handle 40 is in thepredetermined position. In yet other embodiments, all heating elements46 may be reduced in power when downfeed handle 40 is in thepredetermined position.

The heating elements 46 corresponding to one or more of the zones may bereduced in power when downfeed handle 40 is in the predeterminedposition in order to maintain a desired thermal profile along the lengthof muffle 20. As more and more preform 50 is consumed by the drawingprocess and downfeed handle 40 moves closer to lower heater 60, there isa potential risk that the temperature within muffle 20 may increase dueto the combined heat from heating elements 46 and lower heater 60. Ifthe temperature within muffle 20 becomes too high, downfeed handle 40may overheat, which may cause such unwanted side effects such asstretching of the handle. Therefore, the power of one or more of thezones 41, 42, 43, 44 is reduced when downfeed handle 40 reaches thepredetermined position in order to prevent such overheating.

Regulating the temperature of one of more of the zones on downfeedhandle 40 may also maintain the temperature of upper muffle extension 23such that the Grashof number of draw furnace 10 is maintained below acritical value required for steady convection. For example, the Grashofnumber may be maintained within the range of about 800 to about 1200.

As discussed above, heating elements 46 are coupled to downfeed handle40 to reduce flow instabilities in the process gas, thus allowing suchgases as nitrogen and argon to be used. Additionally, by creating theheating zones, overheating of downfeed handle 40 may be prevented. Thedifferent heating zones may also help to better regulate temperaturewithin muffle 20, allowing, for example, relatively lower temperaturesealing materials to be used. For example, seal 26 may be comprised ofrelatively lower temperature sealing materials such as silicones,polyurethanes, rubber, or other elastomeric materials.

As also discussed above, heating elements 46 consecutively heat zones41, 42, 43, 44 once the zones are each disposed, at least in part,within upper muffle extension 23. However, it is also contemplated thateach zone is heated when the zone is disposed, at least in part, withintop hat 21. Therefore, for example, the second position of downfeedhandle 40 may be when first zone 41 is disposed, at least in part,within top hat 21.

FIG. 5 depicts another embodiment in which heating elements 46 arereplaced with a non-contact heating element(s). In this embodiment, forexample, induction heating provides non-contact heating to heat zones41, 42, 43, 44. As shown in FIG. 5 , an induction coil 100 surroundsmuffle 20 and generates a magnetic field that couples with downfeedhandle 40. The magnetic field passes through the material of uppermuffle extension 23 and heats downfeed handle 40. For example, downfeedhandle 40 may be comprised of graphite and upper muffle extension 23 maybe comprised of quartz in the embodiment of FIG. 5 .

FIGS. 6A-13 present results of computational fluid dynamic (CFD)simulations in terms of plots of gas flow and temperature within afurnace assembly (e.g., draw furnace 10). The CFD simulations werevalidated using historical observations based on current productionconfigurations to confirm their validity.

Referring to FIGS. 6A and 6B, depicted are CFD models showingtemperature distributions and gas flow patterns for three examplesduring a drawing procedure. Specifically, Comparative Example A shows amodel of an unheated handle with helium as the process gas, ComparativeExample B shows a model of an unheated handle with argon as the processgas, and Example C shows a model of a heated handle with argon as theprocess gas. In all three examples, a downfeed handle was used with anouter diameter of 4.87 inches and an inner diameter of 4.49 inches.Additionally, for Example C, the downfeed handle was heated to about800° C. using heating elements 46 and insulation with a thickness of2.565 inches was added to the exterior wall of the upper muffleextension for a length of 47 inches.

As shown in FIG. 6A, a comparison of Comparative Examples A and B withExample C shows that the purge gas of Example C has a higher temperaturein the annular space between the downfeed handle and the upper muffleextension (e.g., area X). For example, the purge gas in the annularspace of Comparative Examples A and B has a temperature of about 75-100°C. Conversely, the purge gas in the annular space of Example C has atemperature of about 600° C. The increased temperature of Example C isdue to the heating elements on the downfeed handle.

As shown in FIG. 6B, in Comparative Example B, which uses an unheateddownfeed handle and argon as the process gas, a multi-cellularbuoyancy-driven flow pattern is established in the annular space in anupper portion of the furnace. More specifically, the flow of the processgas is unstable and time variant, resulting in temperature and pressurefluctuations within the annular space. Comparative Example A, which useshelium as the process gas, has consistent gas flow with only two smallrecirculation vortices formed near the inlet of the gas due to the gasentering the furnace perpendicular to the wall. Thus, ComparativeExample A provides a stable, time invariant, flow of process gas.Similar to Comparative Example A, Example C also provides a stable flowof process gas. However, Example C is able to achieve the stable flowwhen using argon as the process gas by heating the downfeed handle. Morespecifically, FIG. 6B shows that the buoyant flow of Example C is stableand temperature fluctuations are suppressed.

FIG. 7 shows a plot of gas temperature as a function of time at locationX for Comparative Examples A and B and Example C. As discussed above,Comparative Example A, which uses helium as the process gas, has arelatively stable temperature plot. Similarly, Example C, which uses aheated downfeed handle and argon as the process gas, has a relativelystable temperature plot. However, Comparative Example B, which uses anunheated downfeed handle and argon as the process gas, has a relativelyunstable temperature plot. Comparative Example B has a large temperaturefluctuation ranging from about 150-400° C.

FIG. 8 shows a plot of gas pressure as a function of time at location Xfor Comparative Examples A and B and Example C. Similar to thetemperature plots discussed above, Comparative Example A and Example Chave relatively stable gas pressures at location X. Conversely,Comparative Example B has a relatively unstable gas pressure at locationX.

As discussed above, the temperature and gas fluctuations near uppermuffle extension 23 can be propagated downward within muffle 20 to theneckdown region of preform 50. FIG. 9 shows a plot of gas temperature asa function of time at location Y (near the neckdown region of preform50) for Comparative Examples A and B and Example C. Due to the use ofhelium and a heated downfeed handle, respectively, Comparative Example Aand Example C have relatively stable non-fluctuating temperature plots.Comparative Example B again has a relatively unstable temperature plot,with temperatures fluctuating from about 1771° C. to about 1816° C. overa 50 second interval.

FIG. 10 shows a plot of gas pressure as a function of time at location Yfor Comparative Examples A and B and Example C. Similar to FIG. 8 ,Comparative Example A and Example C have relatively stable gaspressures, while Comparative Example B has a fluctuating gas pressure.

FIG. 11 shows a plot of temperature as a function of time at location Yfor three different heat rates of downfeed handle 40. More specifically,a first downfeed handle was heated to a temperature of about 400° C., asecond downfeed handle was heated to a temperature of about 600° C., anda third downfeed handle was heated to a temperature of about 800° C. Thefirst downfeed handle temperature produced the most temperaturefluctuations at location Y, and the third downfeed handle temperatureproduced the least temperature fluctuations at location Y. Therefore,heating a downfeed handle to 800° C. produces a more stable temperatureat location Y (near the neckdown region of preform 50) of a mufflecompared with heating the downfeed handle to a temperature of 600° C.and to a temperature of 400° C.

An FFT (Fast Fourier Transform) analysis of the data of FIG. 11 is shownin FIG. 12 . As shown in FIG. 12 , the amplitude of the temperaturefluctuation decreases with increasing temperature of the downfeedhandle. FIG. 12 shows that the fluctuation is significantly suppressedwhen the temperature of the downfeed handle approaches about 800° C.

Similar to the plots of temperature vs. time, gas pressure fluctuationsat location Y are also relatively more stable with a temperature of thedownfeed handle approaching about 800° C. FIG. 13 shows a plot of gaspressure as a function of time at location Y for the three downfeedhandle temperatures: the first downfeed handle heated to a temperatureof about 400° C., the second downfeed handle heated to a temperature ofabout 600° C., and the third downfeed handle heated to a temperature ofabout 800° C. The first downfeed handle temperature produced the mostgas pressure fluctuations at location Y, and the third downfeed handletemperature produced the least gas pressure fluctuations at location Y.Therefore, heating a downfeed handle to 800° C. produces a more stablegas pressure at location Y (near the neckdown region of preform 50) of amuffle compared with heating the downfeed handle to a temperature of600° C. and to a temperature of 400° C.

As discussed above, the optical fiber draw furnace disclosed hereinadvantageously allows process gases with lower kinematic viscosity (suchas nitrogen or argon) to be used while still providing a drawn opticalfiber with a constant and uniform diameter.

What is claimed is:
 1. An optical fiber draw furnace system comprising:a muffle comprising an upper muffle extension and forming an innercavity; a downfeed handle moveably positioned within the inner cavity;and an upper heater comprising one or more heating elements moveablewith the downfeed handle within the inner cavity, the one or moreheating elements comprising a first heating element and a second heatingelement, the first heating element configured to heat the downfeedhandle separately from and independently of the second heating element.2. The furnace system of claim 1, further comprising a lower heaterdisposed within the furnace, the downfeed handle being moveable relativeto the lower heater.
 3. The furnace system of claim 1, wherein the oneor more heating elements are coupled to the downfeed handle.
 4. Thefurnace system of claim 3, wherein the one or more heating elements aredisposed on the downfeed handle.
 5. The furnace system of claim 3,wherein the one or more heating elements are disposed on an inner wallsurface of the downfeed handle.
 6. The furnace system of claim 1 whereinthe one or more heating elements are disposed radially inward of themuffle.
 7. The furnace system of claim 1, wherein the first heatingelement and the second heating element are positioned along a length ofthe downfeed handle.
 8. The furnace system of claim 1, wherein the firstheating element is configured to be in an on position while the secondheating element is in an off position.
 9. The furnace system of claim 1,wherein the one or more heating elements are configured to sequentiallyand separately heat a plurality of zones of the downfeed handle.
 10. Thefurnace system of claim 1, further comprising a gas inlet portpositioned in the upper muffle extension and configured to injectprocess gas in the inner cavity.
 11. The furnace system of claim 1,further comprising insulation disposed around the upper muffleextension.
 12. The furnace system of claim 11, wherein the insulationextends in length from a lower heater, disposed within the furnace, tothe upper muffle extension.
 13. The furnace system of claim 1, whereinthe one or more heating elements are configured to heat at least aportion of the downfeed handle to a temperature ranging from about 400°C. to about 1000° C.
 14. The furnace system of claim 13, wherein the oneor more heating elements are configured to heat at least a portion ofthe downfeed handle to a temperature of about 800° C.
 15. The furnacesystem of claim 1, further comprising a process gas in the inner cavity,the process comprising at least one of nitrogen and argon.